1. Technical Field
The present invention relates generally to transistor amplifiers and, more particularly, to low noise wideband direct-coupled microwave amplifiers which employ bipolar transistors such as a common-emitter transistor and Darlington-connected pair of transistors with a feedback topology.
2. Discussion
Broadband low noise direct-coupled microwave amplifiers are increasingly being required to serve the needs for a variety of modern electronic applications which operate over a wide frequency range extending from DC to microwave frequencies. Some applications include instrumentation systems, radio and cellular phone systems, personal communication networks (PCN), high definition television (HDTV), cable television (CATV), optical communications, and global positioning satellite (GPS) systems. These and other applications commonly operate over wide frequency bandwidths which may cover a frequency range from zero hertz (i.e., DC) extending to over five Gigahertz. For example, zero to five Gigahertz transimpedance amplifiers are particularly useful with digital optical communication receivers which operate at data rates up to ten Gigabits per second (10 Gbit/s) and faster. In another example, direct-coupled microwave amplifiers are useful for oscilloscope applications which operate at microwave frequencies.
These and other applications generally require what is known as a direct-coupled amplifier design topology which can achieve broadband low noise performance and operate into the microwave frequency range. In the past, low frequency performance below 100 Megahertz has been achieved with the use of silicon and GaAs bipolar technology. This essentially precludes the use of HEMT or FET technology which is more commonly used to achieve lower noise levels, but typically only at frequencies exceeding a few hundred Megahertz. This is because state of the art silicon technologies operating at high frequencies generally do not offer low noise performance over an extended frequency range. However, modern and future applications such as HDTV increasingly require wideband low noise performance over a broad frequency range such as 45 Megahertz to 2.5 Gigahertz, for one example.
Broadband direct-coupled amplifiers are commonly implemented with a feedback topology and generally contain a number of transistors, resistors and level-shifting diodes. The conventional common-emitter feedback topology typically employs a single transistor. In bipolar technology, a popular wideband direct-coupled topology is the Darlington feedback amplifier. The Darlington feedback amplifier includes a Darlington-connected pair of bipolar transistors with internal feedback characteristics which generally allows for the realization of wider bandwidths.
The conventional Darlington feedback amplifier can typically achieve broad frequency bandwidths which cover a range as high as eighty percent of the cut-off frequency for a number of existing microwave devices. These types of wideband amplifiers are commonly designed in single and multiple stages and require proper adjustment of parallel and series feedback resistors so as to realize added feedback. However, the conventional broadband amplifier approaches commonly utilize feedback paths which generally introduce additional unwanted noise into the amplifier output. In other words, conventional wide bandwidth amplifiers such as the Darlington-connected feedback type are generally achieved at the expense of realizing higher amounts of noise. Also, these conventional amplifier approaches usually fail to optimize source impedance matching for microwave devices which results in added noise.
It is therefore desirable to provide for a microwave amplifier which may operate over a wide frequency bandwidth while sustaining minimum noise interference. It is also desirable to provide for such an amplifier which may be designed so that gain-bandwidth performance and noise interference may be adjusted independent of one another. More particularly, it is further desirable to provide for a two-stage direct-coupled microwave amplifier which has a Darlington-connected pair of transistors directly coupled to a common-emitter transistor in a manner which provides a wideband low noise direct-coupled amplifier, and which does not adversely suffer from the adverse drawbacks which presently exist with many of the conventional approaches.